You can use HTML to limit the amt of chars someone types in.
<input type = text name=something value=1 size=20 maxlength = 15>
Or you could use javascript to check Onblur is the size of the text box greater than 15. On the server side, to protect yourself, do something like this:
myvar = mid(myvar, 1, 15) (Thisll truncate that to only 15 characters. Hope this helps